AbstractSoluble phenol formaldehyde resin containing hydroxymethyl groups has been used to modify sulfonated poly(ether ether ketone) (SPEEK). Modification has been carried out with films containing both the polymers and using dimethyl formamide (DMF) as casting solvent at various temperatures under reduced pressure. Associated solvent and the hydrogen‐bonded by‐product dimethyl amine (DMA) were removed through mild alkali–acid–water treatment. Cured and treated films show good and consistent mechanical properties, water uptake (22–25%), ion‐exchange capacity (1.1–1.5 meq/g) and proton conductivity (125–150 mS/cm) at 30°C and hold promise for application in fuel cells, as indicated by a polarization study in a fuel cell test station. Copyright © 2007 John Wiley & Sons, Ltd.